Note.. you don't need to be 13 to play the game.
But, If you use steam, than you might be dealing with money transfers, and obviously, there is a minimum age that goes with handling money online.
So to try to get a parent or guardian to make sure everything goes well with younger clients, they have that in place.
If you can just tick a box, sure then who is to know. But if lateron something goes wrong, then the parent or guardian has responsibility, and not the people at Steam.
So it's a protection that works both ways.